The present invention relates to a cooling system for an engine.
Conventionally, known cooling systems for vehicles form a plurality of coolant flow paths passing through an engine body (cylinder head or cylinder block) or auxiliary machinery (heater core, exhaust gas recirculation (EGR) device, etc.), and are provided with a flow rate control valve for controlling coolant flow rates of the respective coolant flow paths (e.g., JP2013-224643A). Such a cooling system restricts the flow of the coolant into the engine body by the flow rate control valve while the engine is being warmed up after a cold start (in a cold state) so as to stimulate a temperature increase of the engine body. When the temperature of the engine body becomes high (in a warmed-up state), the cooling system cancels the flow restriction of the coolant into the engine body so as to cool the engine body.
Further, the cooling system of JP2013-224643A effectively reduces nitrogen oxide (NOx) by cooling EGR gas with an EGR cooler to reduce air to be introduced into the engine body. While the engine is being warmed up after the cold start, the flow of the coolant through the coolant flow paths which pass through the cylinder head and the EGR cooler is restricted, and when the temperature of the engine body becomes high, the coolant flow restriction is canceled.
However, in the engine to which the cooling system of JP2013-224643A is applied, since the coolant is not flowed into the EGR cooler in the cold state of the engine, if the EGR gas is introduced into the EGR cooler in the cold state of the engine, the temperature of the coolant within the EGR cooler is increased to the extent that it boils, which may damage the EGR cooler. As a solution, it can be considered that in the case where the EGR gas is introduced, the coolant is flowed into the coolant path which passes the EGR cooler; however, in this manner, the cylinder head is also cooled along with the EGR cooler, and the temperature increase of the cylinder head cannot be stimulated.